docs(changelog): add missing v7 breaking change

`package.json#engines` was changed from v10+ to v18+ in 7.0.0

PR-URL: #435
Credit: @serhalp
Close: #435
Reviewed-by: @isaacs

Author: serhalp

CHANGELOG.md

@@ -24,6 +24,7 @@
 
 ## 7.0
 
+- Drop support for node <18
 - Rewrite in TypeScript, provide ESM and CommonJS hybrid
   interface
 - Add tree-shake friendly exports, like `import('tar/create')`